A variable displacement compressor is used in air conditioning systems installed in vehicles for air-conditioning. Such type of compressor is disclosed in Japanese Unexamined Patent Publication No. 63-16177.
The publication discloses a compressor which is shown in FIG. 5 of the present application and to which attention should be directed. The compressor has a housing 51 in which a crank chamber 52 is formed. A drive shaft 53 is rotatably supported in the crank chamber 52. A rotor 54 is secured on the drive shaft 53, and a rotary swash plate 55 is rotatably and swingably supported on the drive shaft 53. The rotary swash plate 55 is coupled via a hinge mechanism 56 to the rotor 54. The hinge mechanism 56 consists of an elongated hole 54a provided in the rotor 54 and a pin 56a. The pin 55a is attached to the swash plate 55 and is engaged with the elongated hole 54a. The swash plate 55 is coupled to the rotor 54 and swingable within the range of the length of this elongated hole 54a. An undulation plate 57 is attached to the swash plate 55 with its rotation restricted.
A plurality of bores 58 are formed in the housing 51. A piston 59 is placed in each bore 58. The piston 59 is coupled to the undulation plate 57 and reciprocates within the corresponding bore 58 based on the undulation of the plate 57. A suction chamber 60 is formed adjacent to each bore 58 in the housing 51. A fluid (refrigerant) is supplied to each bore 58 from the suction chamber 60. Likewise, a discharge chamber 61 is formed adjacent to each bore 58 in the housing 51. The fluid compressed by the pistons 59 in the respective bores 58 is discharged into the discharge chamber 61. Formed in the housing 51 is a fluid passage 62 which communicates the crank chamber 52 with the suction chamber 60. Provided in the suction chamber 60 is valve means 63 which senses the pressure in the chamber 60 and adjusts the opening of the bleed fluid passage 62 in response to the pressure.
The thus constituted compressor functions as follows. As the valve means 63 operates in response to the suction pressure in the suction chamber 60, the opening of the bleed passage 62 is adjusted. At this time, the pressure in the crank chamber 52 varies from time to time by the blow-by gas leaking from each bore 58. This pressure change alters the force acting on the back of the associated piston 59 and the balancing point of the moment that acts on the rotary swash plate 55, thus changing the inclination angle of the swash plate 55 and the undulation plate 57. The stroke of each piston 59 changes due to the angular change, so that the compression displacement of the fluid in each bore 58 is changed, controlling the amount of the fluid led into the bore 58. The suction pressure in the suction chamber 60 is controlled so as to be a predetermined value by the mechanism which varies the compression displacement in this manner.
According to the aforementioned variable displacement type mechanism, as the suction pressure in the suction chamber 60 falls due to a decrease in the thermal load in the air conditioning system, the valve means 63 is operated to reduce the opening of the fluid passage 62. The pressure increase in the crank chamber 52 is accelerated to control the compression displacement of the compressor in the direction of reducing it. When the thermal load further decreases, the valve means 63 is operated to completely close the fluid passage 62, so that the pressure in the crank chamber 52 further rises. This further reduces the compression displacement.
Even in this case, the reduction of the compression displacement is restricted to a predetermined minimum value. This is because that in an extremely small displacement area where the compression displacement becomes zero or extremely small close to zero, no effective compression-oriented work is performed. The restoration of the compression displacement, which should be accomplished by the difference between the suction pressure in the suction chamber 60 and the pressure in the crank chamber 52, becomes thus practically impossible. In recent compressors in which the individual sliding portions in the compressors are required to be lubricated with the oil mist admixed in the refrigerant, the burning at the individual sliding portions and the reduction in durability of the compressors due to insufficiency of refrigerant (lubrication) can be pointed as the factors which restrict the minimum value of the variable compression displacement.
So long as the minimum compression displacement is restricted as mentioned above, when a vehicular air conditioning system including a compressor and evaporator is used in the environment of a cold place or the like, the operation of the compressor must be controlled properly to protect the sliding portions of the compressor against wear and prevent freezing of the evaporator. For example, the compressor should be properly stopped by cutting off the power transmission to the compressor by an electromagnetic clutch. The electromagnetic clutch coupled to the compressor is widely used as an essential component of the current vehicular air conditioning systems.
With the use of a vehicular air conditioning system that employs an electromagnetic clutch, however, the shock at the time the system is activated thereby affects the driving feeling of the vehicle. In addition, the alternator for supplying power to the electromagnetic clutch has a surprisingly low efficiency, increasing the engine load accordingly, which is not negligible, either. In other words, it is easily understood that the elimination of the electromagnetic clutch, if possible, can significantly reduce the weight of the vehicular air conditioning systems as well as can contribute to reducing the fuel consumption.
It is therefore an object of the present invention to ensure protection of the individual sliding portions of a compressor against wear and suppression of over-cooling or the like, while eliminating the electromagnetic clutch.